The present invention relates to internal combustion engines and in particular to an apparatus and method for varying the valve timing pertaining to a combustion chamber thereof.
As known in the art, the "dwell" of a valve is the portion of the camshaft rotation cycle during which the valve is open. As further known in the art, the opening and closing of a valve are referred to as the valve events. The dwell occurs between these two valve events. The events of a rotation cycle can be defined in terms of engine crankshaft degrees. Accordingly, the valve events occur at particular angles, and the dwell can be defined as an angle extending between the two valve events. In an engine with dual camshafts, one camshaft actuates the intake valves, and the other camshaft actuates the exhaust valves. However, in a single-camshaft engine, both intake and exhaust valves are actuated by the single camshaft.
The sequence of the valve events which defines the period during which the intake and exhaust valves are both open determines what is known in the art as the "valve overlap period," also spoken of in terms of a "valve overlap angle." Specifically, the valve overlap period is the period between when the intake valve opens and the exhaust valve closes. In other words, the portion of an engine revolution from the opening of the intake valve to the closing of the exhaust valve is known as the valve overlap angle.
Improvements in power output, economy, and emissions of spark-ignition engines are obtained by variable valve timing which involves changing the timing of one or more of the intake and exhaust valve events. Variable valve timing also provides benefits for diesel engines, including: improved starting, the use of a lower compression ratio, reduction in diesel "knock," the ability to use lower quality fuels, a raising and flattening of the torque curve, improved fuel consumption, reduced emissions, and better control of scavenging in turbocharged engines.
The induction and exhaust systems of internal combustion engines are designed specifically for the type of operation that the engine is expected to perform. An important function in these systems is accomplished with the intake and exhaust valves, since cyclically opening and closing these valves allows for four-stroke operation. The timing of valve actuation is accomplished with one or more camshafts and is determined by the angular relationship between the camshaft(s) and crankshaft. This timing is critical for proper breathing characteristics, but is fixed at what is considered optimum for the expected utilization of the engine. Because of the inertia effects of the gases being inducted into and expelled from the cylinders, the valve timing is considerably different for the various classes of engines. For example, the family automobile has valve timing optimized for ordinary highway operation, and at other speeds the engine performance is less than ideal. The current technology of fixed-valve timing allows for a simple, rigid and compact camshaft and drive arrangement. This system is limited, however, since the valve opening and closing angles are compromised over the engine's entire load and speed range.
One means for achieving improved performance is through the use of intake-valve control. The valve-overlap period, i.e., the period between when the intake valve opens and the exhaust valve closes, has a significant influence on engine breathing and performance characteristics. Operation of a high-performance engine at rated speed requires a large valve-overlap period to compensate for gas inertia effects. Since these effects are minimal at low speeds, the engine generates its maximum load-speed torque with a small valve-overlap period. In order to meet both of these objectives, various control systems have been designed with the capability to provide variable overlap periods.
Numerous different variable valve timing mechanisms have been tried. Most of these designs accomplish variable overlap by shifting, i.e., phasing, of the intake camshaft relative to the exhaust camshaft to change the angle between when the intake valve opens and the exhaust valve closes. Thus, camshaft phasing involves changing the angular relationship between one or more of the engine's camshafts and the crankshaft. Several automotive manufacturers are adopting camshaft phasing as a limited form of valve timing control for their engines. Camshaft phasing has been used primarily for changing the valve-overlap angle and has been adopted almost exclusively for this use on the intake camshaft of engines with twin camshafts. Variable ovelaps allow for improved performance, improved fuel consumption and lower emissions. In most applications, the valve overlap angle is varied to achieve a small overlap at low-speed and tight-load conditions and large overlaps at high speeds and loads.
Many camshaft-phasing mechanisms are being proposed for the marketplace. Most units are used to vary the phasing of the intake camshaft alone, but could be applied readily to any camshaft. Some of the phasing mechanisms are controlled by infinitely variable positioning devices, and most are electro-mechanically or electro-hydraulically controlled. Toyota's FX-1 concept car uses stepper-motor control to adjust continuously the phasing angles (i.e., relative to the crankshaft) of both the intake and exhaust camshafts by 10 degrees. Renold-of-Britain's design is hydraulically controlled and adjusts timing over a 15-degree range relative to the crankshaft. Alfa Romeo introduced into production a two-stage phase adjuster which can advance a camshaft by a fixed angle of 16 degrees with no intermediate angles. This device has a solenoid-actuated hydraulic control which is activated as a function of load and speed. Nissan's valve control system similarly makes a 14-degree change in the valve opening and closing angles by phasing the intake camshaft relative to the crankshaft. It is also a two-stage mechanism which is hydraulically controlled and solenoid actuated.
U.S. Pat. No. 4,388,897 to Rosa, proposes a variable valve timing device comprising a camshaft wormed over part of its length and carrying a splittable cam assembly separable along the axis of rotation of the camshaft. However, in the Rosa device the control over the valve event is dependent upon the speed of the camshaft. Moreover, the degree of control over the valve event permitted by the Rosa device is limited by the sensitivity of the linkage that restrains axial movement of the camshaft. Furthermore, the axial shifting principle of the Rosa device renders it difficult to miniaturize to conserve space in the engine compartment.
One problem with camshaft phasing is that the dwells remain constant and the intake valve's closing angle may be compromised when the opening angle is varied. This compromising effect becomes more pronounced for a camshaft having both intake and exhaust cams. This is because intake closing, exhaust opening, and exhaust closing angles all are potential tradeoffs. For this reason, camshaft phasing for the purpose of controlling valve-overlap periods is restricted at present to phasing the intake camshaft on engines having dual camshafts. Thus this expanded technology carries with it inherent disadvantages which limit the benefits to be achieved through its use.
At low spark-ignition engine speeds, performance optimization dictates a small overlap angle to reduce the likelihood that intake and exhaust gases mix. Gas-flow dynamics preclude mixing at high engine speeds, and therefore a large valve overlap angle is desirable to achieve greater volumetric efficiency and the corresponding increase in power output. If the valve overlap angle can be varied during the operation of the engine, then the flow rate into and out of the engine can be maximized both at high speeds and for full-load operation at any speed. Increasing the valve-overlap angle for the low- and mid-speed ranges is beneficial for emissions control because of the mixing of intake gases and exhaust products. However, this same increase in valve-overlap angle for the low- and mid-speed ranges hurts power output and economy.
Improving the fuel consumption efficiency of an internal combustion engine can be accomplished by decreased friction, higher compression ratios, improved combustion, and reduction of an engine's pumping losses. The pumping losses, which are the negative work required by an engine to intake and exhaust gases during operation, are a significant fraction of the losses which reduce the fuel consumption efficiency of the engine. In the case of a spark-ignition engine, these losses result primarily from the resistance associated with the flow of fresh air past the throttling valve before entering the individual combustion chambers of the engine. The throttle performs the necessary function of controlling the engine power output by varying the amount of air/fuel mixture available for consumption. Thus, any elimination of the throttle valve for the purpose of reducing pumping losses requires an alternative means of controlling the amount of air/fuel mixture inducted into the combustion chamber to support the required engine load.
A standard spark-ignition automobile engine operates the majority of the time at part throttle where pumping losses are greatest. The penalty in part-load performance of a conventional spark-ignition engine varies from 3.5% of the nominal mean-effective pressure at wide-open throttle to nearly 100% for a fully throttled idling engine. These performance penalties are attributed primarily to the throttling process. It is believed that running an engine at wide-open throttle throughout its load-speed range would improve the average overall efficiency of the engine by about 20%.